1-Bṛhan-nāradīya Purāṇa 3.8.126 and
2-Śrīmad Bhāgavatam 6.3.24.
Here it is told to chant the names of the lord but I am confused because I do not know the names? Is it "VishnuSahasraNamam"?
If the « the names » has been explicitly mentioned, then obviously it is inferring to Viṣṇu Sahasrāra Nāma since the source is a Vaiṣṇava grantha.
Otherwise if « the name » is mentioned, it is implying to chant one name (to whichever appeals to you).
A note on technical jargon:
Your question leads to a profound topic that was deeply elaborated upon by Sri Caitanya Mahaprabhu. His whole life was dedicated to revealing the 'yuga dharma' [recommended process for gaining liberation] to the whole world, namely the chanting of the holy names of God. Lord Caitanya Himself summarized His mission in Eight Verses called Sri Sikshastaka - www.tiny.cc/siksastaka In the second verse, He explains that all the names of the Supreme Lord are imbued with ALL the potencies/shaktis of God. This is because, out of His causeless mercy, He wants to make it really easy for us suffering Jivas to directly contact, approach and reach Him, for eternal solution to all our problems (birth-death-disease-old-age + tri tapas of sufferings born of our own minds, from the minds of others and from the minds of the devatas/universal administrators) . So those 8 verses give a brief summary of the philosophy and teachings of Lord Caitanya.
Summary essays of His amazing life and teachings are: A. Two essays in the front matter of this book 'The Teachings of Lord Caitanya' which had also been published under the title 'The Golden Avatar' - www.bit.ly/krishnapath-tlc (audiobook and ebook) B. The introduction to the first canto of the Srimad Bhagavatam, which contains an essay titled, 'A Short Sketch Of The Life And Teachings Of Lord Caitanya, The Preacher Of Srimad Bhagavatam'
A summary study of Lord Caitanya's life is the book Teachings Of Lord Caitanya, where His teachings are grouped by Topic (www.bit.ly/krishnapath-tlc)
And a detailed biography of the Lord is the Caitanya Caritamrta by Srila Krishnadas Kaviraj Goswami - www.bit.ly/krishnapath-cc (Two other authorized biographies of Lord Caitanya are Sri Caitanya Mangal by Locan Das Thakur and Sri Caitanya Bhagavata (renamed as such to remove confusion - earlier this book had also been known as Caitanya Mangal) by Vrndavan Das Thakur
Lord Caitanya wanted to glorify His great devotees, so He never wrote anything other than the 8 verses of the Siksastaka. Whichever topic He wanted to teach, He would empower a qualified devotee to act as Acharya for that topic, and He would guide and inspire that person to compose and write the teachings as He wished to present them to the world. Thus for the Science of the Holy Name, he selected the muslim-born Haridas Thakur (incarnation of Lord Brahma according to Sri Gaura Gannodesha Dipika) and Lord Caitanya conferred upon Haridas the glorious title of Nama Acharya. They had a discussion on the science of the holy name. It was composed as a Bengali Poem by Thakur Bhaktivinode (Kedaranath Datta):- www.tiny.cc/hnc - Harinama Cintamani English www.tiny.cc/harinamacintamani - Harinama Cintamani orignal begali poetry This book is a glorious discussion between Lord Caitanya and the Nama Acharya Srila Haridas Thakur, on the topic of how jivas born in Kali yuga may easily attain the perfection of life.
These references in the Bhagavatam may help too - https://vaniquotes.org/wiki/SB_12.03.52_krte_yad_dhyayato_visnum..._cited Especially relevant could be the very last verse of the 18,000-verses of the Bhagavatam - https://vanisource.org/wiki/SB_12.13.23 , which translates: 'I offer my respectful obeisances unto the Supreme Lord, Hari, the congregational chanting of whose holy names destroys all sinful reactions, and the offering of obeisances unto whom relieves all material suffering.'
http://www.krishnapath.org/chanting-hare-krishna-maha-mantra/ - vedic references for chanting specifically the Hare Krishna Maha Mantra
In conclusion, we hope that the wealth of research and practical demonstration via the lilas recorded in the Caitanya Caritamrta (also known as the Darshan Shastra by Acaryas in the line of disciplic succession coming from Lord Caitanya) and other biographies of the Lord, which we have shared above, will help one get a simple and also deeply profound answer about the names of the Lord and which to chant.
Apart from the brief mention we gave above that All of God's potencies are present in Every name of the Supreme Lord, God Himself is personally present in every one of His holy names.
It is revealed by Nama Acarya Haridas Thakur in the HNC mentioned above that the names of our Lord can be divided into two categories - the functional names (which describe His attributes and powers etc) and the personal/lila names (which are imbued with His loving exchanges with His devotees and which flood the hearts of premi bhaktas with prema). Of the two, those on the vaishnava path strongly recommend the latter names as being more efficient in evoking and evolving our spontaneous loving relationship with the Supreme Lord. Hare Krishna!
Hope these resources may be useful to all sincere seekers. Gaura-hari-bol!
B. Gita 10:25 - 'yajñãnām japa-yajño 'smi' = 'of all sacrifices, i am the chanting of the holy names, japa'